The initial argument presents a causal explanation for why more pedestrians are struck by cars in crosswalks than outside of them: pedestrians develop a false sense of security in crosswalks and become less cautious. To most effectively undermine this explanation, you need to find an alternative reason for the observed fact that is at least as plausible as the one offered.
Option A does this directly:
A. The overwhelming majority of pedestrians in high-traffic areas cross streets in crosswalks.
Here's a breakdown of why this most undermines the explanation:
The initial argument's logical flaw: The original explanation jumps to a causal conclusion without considering the base rate of behavior. It observes that crosswalks have more accidents and concludes that crosswalks themselves cause the increased risk through a sense of security.
Alternative explanation: Option A provides a crucial piece of missing context. If most people cross at crosswalks, it's statistically predictable that most accidents would also occur there. This is a simple issue of frequency, not necessarily risky behavior.
Undermining the causation: This new information removes the need for the "false sense of security" explanation. The correlation between crosswalks and accidents is instead explained by the high volume of pedestrian traffic in crosswalks. It suggests that accidents in crosswalks might actually be less frequent on a per-crossing basis than accidents outside of them.
Why other options are less effective
B. The number of pedestrians struck by cars has increased in recent years. This provides general information about a trend but does not explain the discrepancy in accident locations (in vs. out of crosswalks).
C. Pedestrians tend to underestimate the chances that the signals at a crosswalk will malfunction. This statement, if true, would strengthen the original argument, not undermine it. It reinforces the idea that pedestrians are overconfident in the safety of crosswalks.
D. Drivers are generally most alert to pedestrians who are in or near crosswalks. The original argument focuses on pedestrian behavior. While driver alertness is a variable, this statement doesn't change the central logic about pedestrian overconfidence. It might even be seen as a mitigating factor that should make crosswalks safer, making the original observation more puzzling.
E. Measures intended to promote safety tend to make people less cautious. This is a restatement of the original explanation in more general terms. The original argument is an example of this phenomenon, so it does not undermine the specific reasoning